Experiments have been performed on microglia, the resident macrophage in the central nervous system, cultured from cerebral cortices of rat. We have previously found that the calcium ion can enter the microglial cells when these cells are hyperpolarized. We have now found, using patch-clamp electrodes, that there are L-type voltage-dependent calcium channels that become open when cells are depolarized. We have also found that a concentration of 25 uM ferrous ion is toxic to microglia after 3 hours. Since we have previously shown that microglia can produce the superoxide radical ion, we believe that the mechanism of ferrous ion toxicity is due to the enhanced iron catalytic production of the hydroxyl radical from the superoxide radical ion.